The present invention is related to frequency multiplying and, more particularly, to a circuit and method for producing output pulses which occur at twice the rate of input pulses supplied to the circuit.
Frequency doubling circuits are well known in the art. For example, a frequency doubling tachometer circuit is known that comprises a Norton amplifier to which voltage pulses are supplied and converted to current pulses through a series connected RC network which is coupled to the non-inverting input of the amplifier. The output of the amplifier is returned to the inverting input via a parallel RC network. A diode is connected between the inverting and non-inverting inputs of the amplifier. The operation of the circuit is to average the charge and discharge transient currents that flow through the RC network. The capacitor is therefore charged and discharged in response to the leading and trailing edges of the input pulses to produce two current output pulses for each cycle of the input frequency.
The above described frequency doubler circuit has several problems associated therewith that limits its application in systems that require multiple frequency doubling. For instance, control of three phase DC brushless motors wherein a tachometer function is required for each of the three motor phases. A first problem related to the prior art frequency doubling circuit arises due to the fact that the charge and discharge paths for producing the output current pulses are different. This results in the frequency doubling circuit being edge sensitive, i.e., the pulse widths of the two output current pulses will be different which may be highly undesirable in motor control systems. In addition, if multiple frequency doubling circuits are required, all of the individual components of each circuit associated with the motor phases must be matched to one another which is difficult and expensive.
Hence, a need exists for an improved frequency doubling circuit that is suited to be fabricated in integrated circuit form and which is responsive to a plurality of input pulses supplied thereto in phase relationship to one another for producing output pulses at twice the rate that the input pulses occur.